Lasers are ubiquitous devices used for testing, measuring, printing, cutting, marking, medical applications, communications, data transmission, semiconductor processing, and many other applications. Many types of lasers have been developed to meet different performance criteria for different applications. Engraving, cutting, marking, printing, and many other applications require relatively compact lasers that generate high power output and have beams with a desired shape and energy distribution. Gas lasers, such as carbon dioxide (CO2) lasers, are useful in such applications because they can generate continuous, high power output in a relatively compact package.
Carbon dioxide lasers generally include a gas containment structure containing a laser medium, electrodes for providing an electrical discharge within the laser medium, and optics at each end of the containment structure. The laser medium in most CO2 lasers includes a gas mixture of CO2, nitrogen, and helium. The CO2 produces the laser light, the nitrogen helps increase the laser's efficiency by transferring its excitation energy to the CO2 molecules during collisions between the nitrogen and CO2 molecules, and the helium serves to depopulate the lower laser level and remove heat from the laser medium. In some applications, the gas mixture can also include hydrogen (H2), water vapor, xenon, and/or other gases to perform other functions.
One well-known drawback with CO2 lasers is that the electrical discharge that excites the laser medium also causes the CO2 to dissociate into CO and oxygen (O2). More specifically, the energy of the electrical discharge in the gas mixture excites the nitrogen molecules to an elevated oscillating level. During excitation, this stored potential energy is transferred from nitrogen to the oscillating levels of CO2 molecules, which results in a population inversion between the oscillating levels of the CO2. As the CO2 settles from this higher-energy state, photons are released, resulting in laser radiation. The other components of the laser medium (e.g., He and Xe) do not participate in this reaction.
During this reaction, CO2 decomposes into CO and O2, leading to the redistribution of potential energy as the concentration of CO and O2 increases. The dissociation reaction is as follows:CO2+e→CO+O−CO2+e→CO+O+e
Over time, the reaction reaches equilibrium as follows:CO2 □CO+½O2
A portion of the energy of discharge, as well as the oscillating levels of the nitrogen, are transferred to the oscillating levels of these CO and O2 molecules. The probability of the transfer of this energy from the CO and O2 to the CO2 molecules is low. The duration of the vibrational states of the CO and O2 molecules is less than that of the nitrogen. Thus, some of the discharge energy will be transferred to the CO and O2 molecules and will not participate in the creation of the population inversion between the oscillating levels of CO2. Accordingly, the concentration of CO and O2 is inversely related to the power output such that higher concentrations of CO and O2 in the mixture cause lower output power. The graph in FIG. 1, for example, represents laser output power relative to the partial pressure of CO within a CO2 laser with 50-watt rated output power. As the graph illustrates, the output power decreases significantly as the concentration of CO in the gas mixture increases.
One solution to the problem with dissociation has been the use of gold, silver, or other types of metals or metal alloys that provide a catalyst to drive (i.e., oxidize) the CO to CO2. Many conventional systems, for example, include gold layers or coatings distributed on the electrodes and/or the walls of the gas containment structure. One drawback with such conventional approaches, however, is that reactions using the gold-plated electrodes and/or wall portions are unpredictable and difficult to control. For example, reacting the gold-plated portions with the laser medium can take a significant amount of time relative to the laser's discharge reaction time, and it can be difficult to activate the gold catalyst for the reaction between the gold-plated portions and the laser gas medium. In addition, coating the electrodes and/or portions of the walls within the containment structure can be extremely expensive and time-consuming. Accordingly, there is a need to improve the systems and methods for operating CO2 lasers.